Saturday, April 12, 2008

DNA and Forensics II

What is DNA?
Of course, these criminal cases could not have been solved without our knowledge of DNA. In more than 100 years of scientific research it has been proven beyond any reasonable doubt that DNA is the genetic substance responsible for heredity in plants, animals and humans.
DNA was discovered by Friederich Miescher in 1869 in Tubingen, Germany, from the nuclei of white blood cells obtained from the pus on bandages of injured soldiers. Today it is comparatively easy to obtain DNA from a biological sample. Briefly, one carefully mashes the sample, removes all solid matter, extracts with phenol, a substance that destroys the proteins but not the DNA, and then precipitates the DNA with alcohol as a gelatinous gel that is centrifuged. At the time of its discovery, Miescher speculated that a unique substance containing phosphorus as well as carbon, nitrogen, oxygen and hydrogen that was derived from the nucleus of the cell must have great significance. He called it nucleic acid, later known as deoxyribonucleic acid or DNA.
Although DNA was generally ignored in relation to the much more numerous proteins, it was eventually proven to be the genetic substance by Avery and his associates working at the Rockefeller Institute in 1944. Its molecular structure, containing three distinct components (base, sugar and phosphate) was established over a period of years, principally by TA Levene at the Rockefeller Inst. and Lord Todd and associates at Cambridge University. The famous double helical structure of DNA was proposed by Watson and Crick in Cambridge in 1952, and was subsequently proven in many wide ranging experiments.
There are ca. 30,000 genes in the human genome (the total of all genes in an organism). Each gene contains thousands of bases in a specific sequence and each gene has a specific function, most of them coding for the production of a specific protein. The genetic code was first described in detail by Marshall Nirenberg in the 1960s and is now known to be essentially universal in all biological organisms on earth, from bacteria to man (there are exceptions, such as organisms in hot undersea vents where sulphur replaces oxygen).
It is known that the sequence of the four common bases in DNA (abbreviated to A, C, G, and T) is what gives it its informational content as the genetic substance. Changes in this sequence of bases are called mutations, and occur naturally and randomly. Evolution occurs as a result of random mutations in the genome over very long periods of time. This can be shown independently of analysis of fossil bones or other criteria, purely from the DNA/gene sequences of past and present living organisms. By careful comparison of the DNA sequences of a given protein or set of proteins over time, one can construct an evolutionary tree, quite independently of other assumptions. (One corollary of this is that one does not need to assume the existence of God to explain the presence of biological diversity; according to Francis Crick, "God is an unnecessary hypothesis.")


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